This invention relates to a phase correction circuit which can reduce the generation rate of phase skips in a circuit which regenerates the carrier from a PSK-modulated signal, without adversely effecting transient response.
In a system called the multiplication system, a bandpass filter with a narrow frequency band can be used for noise suppression. It is already known that a phenomenon called phase skipping or slipping occurs unless a sufficiently narrow frequency bandwidth can be realized for the bandpass filter used in these systems. In a 4-phase PSK system, for example, this phenomenon provides sudden phase changes of 180.degree. or .+-.90.degree. in the regenerated carrier. If the phenomenon occurs in a system wherein demodulation is performed by means of coherent detection, all data in the subsequent period become erroneous. Therefore, the generation rate of the phenomenon must be minimized. The simplest method of obtaining such a minimized generation rate is to make the frequency bandwidth of the bandpass filter used for noise suppression sufficiently small, but this introduces defects. FIG. 1 is a conventional circuit for regenerating the carrier from an N-phase PSK signal. In this figure, 11 is an N-times multiplication circuit; the part surrounded by the dotted line is a circuit called tracking filter 12; 13 is a 1/N divider; 14 is a bandpass filter; 15 and 16 are frequency converters; 17 is a phase detector; 18 is a voltage controlled oscillator and 19 is an N-times multiplication circuit.
It is already well known that if filter 14 has a sufficiently narrow frequency bandwidth, the generation rate of the phase skipping which occurs when the signal frequency is divided by divider 13 will be reduced.
When this circuit is used, for example, in the burst mode wherein the signal is intermittent, the transient response must be taken into account since this circuit employs a feedback control system. In order to improve this characteristic, it is necessary to widen the frequency bandwidth of bandpass filter 14. Therefore, it has been very difficult to produce a filter circuit which simultaneously satisfies both the phase skipping characteristic and the transient response characteristic.